This invention relates to an improved method of pretreating aluminum sheet for the resistance welding thereof.
The principle of resistance spot welding is based on heat generated by electrical interfacial resistance to the flow of electric current between two or more work pieces held together under force by a pair of electrodes which act as electric conductors. Maximum heat is produced at the faying surface (the mating surface of two sheets to be joined) by a short time pulse of low-voltage high amperage current to form a fused nugget of weld metal.
The interfacial resistance of the work pieces is both the promoter of and the limiting factor of the process. Promoter because one requires interfacial resistance at the faying surface to produce a weld. Limiting factor because accumulation of heat generated by the interfacial resistance at the electrode/work piece surface after a number of welds leads to deterioration of the electrode tip. In resistance spot welding, aluminum deterioration of the electrode tip is further accelerated by the inherent physical, mechanical properties and surface condition of the work pieces.
It is a well known and accepted fact that the resistance spot welding weldability of aluminum in the as received mill finish condition is both poor and erratic. The reason for this poor and inconsistent weldability has been associated with the large variation in surface resistance which in turn is related to the nature and non-uniformity of the oxide layer and to the surface condition. One of the main goals of the aluminum industry over recent years has been to improve the resistance spot welding weldability of aluminum to a level acceptable by the automotive industry as a prerequisite for the use of aluminum in autobody sheet.
Various methods have been suggested over the years for treating the surface of aluminum in preparation for resistance spot welding. For instance, Dorsey, U.S. Pat. No. 4,097,312 issued June 27, 1978 describes the formation of an oxide coating on the aluminum surface and stabilizing this by treatment with a hot aqueous alkaline solution containing long chain carboxylic acids. An arc-cleaning technique is described by R. F. Ashton and D. D. Rager in "An Arc Cleaning Approach For Resistance Welding Aluminum", Welding Journal, September 1976, page 750. In addition, several technical papers have been presented dealing with ways and means of improving weldability.